Passivation of Interfaces in High-Efficiency Photovoltaic Devices
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0)
E 0.6 10.4 -
-•02 0~
----~t - .
0.8-
-.
~~Sp (cm/S)
-" ,
-
-
",,-
-
*..-
",
10
---- 10 1065353
0.)
-0.0
,---2.0
2.5
Photon energy (eV)
I
T
3.0
3.5
Fig. 4 Internal QE calculated using xB = 3 pm, S, =1000 cm/s, and a variable value of Sp. "1n CD
0.8 a)
E 0.6 =•0.4 Cz
"c0.2 •) -Enn V,
2.0
2.5
3.0
3.5
Photon energy (eV) Fig. 5. Internal QE calculated using a variable value of S,. The lower set of curves used xB =0.45 ptm; for the upper set, xB =3 pm.
98
is thinned so that more light penetrates near the back surface, then an increase in S, causes a decrease of both the red and the blue response, with the red response decreasing even more dramatically. The most significant result here is that the QE is increased when S,, or Sp is reduced from 107 to about 104 cm/s, but further reduction of S,, or S, has a negligible effect on the QE. Similar calculations for p-on-n cells show that the effec4of passivation of the back of the cell is less than for the n-on-p cell. In practice, a p-on-n cell usually uses a thicker emitter and thinner base. The differences observed between the n-on-p and p-on-n cell are a result of the poor minority hole transport properties compared with those of electrons. Poor hole collection in a 0.1pm-thick, n-type emitter is not a serious problem in the n-on-p cell, but is much more of a problem in a 3-pm-thick, n-type base of a p-on-n cell. However, in practice, because most GaInP cells are grown thin to help match the currents of the GaInP and GaAs cells [9], both cells require passivation of both the front and the back of the cells. Thick GaAs p-on-n cells benefit negligibly from back-surface passivation because the base thickness > Lp. Passivation of the solar cell also decreases the dark current of the solar cell, and, therefore, increases the Voc. Assuming ideal material, the transport equations can be solved to give the dark current associated with the base and emitter regions (first two terms in eq. 4) [7,8]. An estimation of the dark current in the depleted layer is included as the third term [8] and is discussed below. SIL. hcosh'cosh XB +sinh XB1
r= q /,4. ( M [D,,
J(V)=q -
A
SL
Ln)
_L.
. expqTV-l
sinhX-+cosh 4 D,.
Ln(4)
IL I U n
SpLP (cosh! X coshX-+ sinhXE]
qr
+ q ND LLPJLND_
LP Lh L ep qV_ [m.1r~i1 I h, SPLP sinh xE' Dp LP + cosh xE ____
J
SLexp-_]+[ kTdU
d Lj 2(v-Vrie
2kT n. W. )r .expk- -q -
where ni is the intrinsic carrier concentration, NA and ND are the concentrations of acceptors and donors, Vd is the built-in voltage, and r is the nonradiative carrier lifetime, given by lfNT(vth, the reciprocal of the product of the trap density, the capture cross section, and the thermal carrier
velocity [8]. The base and emitter terms are the diode injection current and are affected both by bulk recombination and by interface recombination. The impact of the two types of recombination on the relative Voc can be expressed as a function of two dimensionless variables [9]. The bulk recombination is described by the ratio of the layer th
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